Getting to the root of hair loss can take some detective work, but here's the bottom line: It's not just a beauty issue.

The Culprit: StressHair loss from so-called delayed stress happens months after a traumatic event like major surgery, a physical illness, the death of a loved one, or divorce.

"That sort of stress shocks a large percentage of hair follicles into the resting phase, meaning they stop producing," says David Bank, M.D., a dermatologist in Mount Kisco, New York. "You don't get hair loss at that moment. It's when hair goes from sleeping back into the growing phase, when you're feeling better, that the new hair comes up and pushes old hair out."

What you can do: Hair loss due to acute stress is almost completely reversible. "Each hair you see in the drain is being replaced by a new one," Bank says. "It's a one-for-one swap. " If you're experiencing ongoing extreme stress, it may help your hair if you add yoga, mindfulness meditation, or other relaxation techniques to your daily routine.

The Culprit: DietIf you deprive yourself of vitamins and nutrients, especially the B vitamins and iron, your hair will suffer. "When you're eating a lousy diet with too much fat, you'll get cholesterol plaque deposits and thinning of the blood vessels that feed the hair follicles," Bank explains. "You're impeding the ability of the hair to get the maximal oxygen and nutrients it needs."

What you can do: Eat a healthy, balanced diet rich in the B vitamins biotin and folate (folic acid). They're found in a variety of foods: leafy green vegetables such as spinach, as well as oranges, rice, beans, peas, carrots, cauliflower, soybeans, bran, nuts, and eggs.

If you're still seeing loss, ask your doctor to check your levels at your next checkup. It may be wise to take a daily B vitamin complex that includes 200 micrograms of folic acid and 2.5 mg or 2,500 mcg of biotin.

The Culprit: Hormones and AgingWonky hormones are one of the biggest reasons women lose hair. The sources include pregnancy, birth control pills, hormone therapy, and thyroid hormones.

"If a woman comes in complaining of increased hair loss, we'll take a medical history," Bank says. "If she just had a baby and she's shedding, it's not usually necessary to dig deeper."

If you're not postpartum, expect to have a full blood panel workup done, which includes a look at your thyroid levels, estrogen, progesterone, and other hormones, including testosterone, cortisol, and DHEA (dehydroepiand-rosterone), a hormone secreted by the adrenal glands.

If, on the other hand, you come from a long line of thin-haired relatives, there's a possibility you will have thinning locks too.

As you age, each hair strand gets smaller in diameter, affecting the overall volume. "It's programmed into our DNA that as we get older, a percentage of our hair follicles will shrink and either make smaller, thinner hairs at a slower rate or stop making hairs altogether."

What you can do: If your hair loss is hormonal, it will grow back. "Hormonal shifts aren't destroying or damaging your hair follicles," Bank says. "The hormones have signaled the follicles to shrink or go to sleep. Your body will shift back to normal once hormone levels stabilize [about three to six months]."

Feeding Your Hungry HairOnce you've found the reason for your hair loss, there are things you can do to make your locks a little more lustrous while you await regrowth.

For starters, make sure your diet is rich in protein, which builds keratin, the hair's outermost layer. Opt for regular servings of fish, lean meats, cheese, eggs, spinach, and soy.

Topical scalp treatments that contain essential oils like chamomile and lavender nourish the skin of the scalp and help prompt hair growth. Weekly scalp massages (at a salon or even in your shower) can also relieve stress and stimulate sebaceous glands.

To keep your hair at its fullest, find a soft, natural style that frames the face, says Younghee Kim, owner of Younghee Salon in New York City. Layered, tousled looks will give your hair an extra boost.

"Treat your locks gently, especially if they're feeling fragile," says Philip Kingsley, founder of the Philip Kingsley Trichological Center in New York City. As hair declines in volume, women often want to resuscitate it with a lot of blow-drying and brushing, but that can be damaging to fragile strands. "Hair is organic," Kim says, "and should be treated with care."